He had back discomfort in the playoffs, but it didn't sideline him. Season ended, and the discomfort went away and he felt fine. He ramped up last month in Arizona, and the discomfort came back. Why would he get a second opinion in December if his back felt fine? Every player has aches and pains at the end of a long season. I guarantee that's what he thought, and what the team thought.
I'm not sure what you are griping about here, but your blame and anger is kind of oddly misplaced. It's a crappy situation. But you make it seem like the team should have done something to avoid it. What could they have done?
The treatment for stress fractures in the back is rest and physical therapy. The fact that Williams isn't healed after the offseason is concerning. But that isn't the team's fault.